Method of exposing light in a method of fabricating a reticle

ABSTRACT

In a reticle, a semi-transparent film pattern in place of a light blocking film pattern is used as a mask pattern having a size within a certain range, whereby an exposure system can be improved in resolution limit and faithful pattern transfer can be realized with a constant light quantity. A reticle may be of a stacked layer structure which comprises a shift film for providing a different optical path to exposure light, a mask substrate formed on the top or bottom of the shift film, and a transmissivity ratio adjustment layer having a predetermined transmissivity ratio to the exposure light. The material of a phase shifter may be adjusted in amplitude transmissivity ratio so that a shifter width for effectively improving a contrast can be made large and an accuracy necessary for a shifter width can be loosened. The mask pattern may include a semi-transparent film pattern which is made of silicon, a silicon compound, a mixture containing silicon, germanium, a germanium compound or a mixture containing germanium to provide a different optical path from that of the transparent part with respect to lithographic light. A silicon compound, a mixture containing silicon, germanium, a germanium compound or a mixture containing germanium is controlled in its composition ratio to form a semi-transparent film pattern on a transparent substrate.

This is a division of application Ser. No. 08/192,091, filed Feb. 4,1994 now abandoned which is a continuation-in-part of application Ser.No. 07/798,721, filed Nov. 29, 1991 now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a reticle and a method of fabricating areticle.

The semiconductor integrated circuit has been steadily increase in theirhigh integration and miniaturization properties. In the fabrication ofthe highly integrated semiconductor circuit, a lithography techniqueplays a vital role.

Most of the lithography techniques now employ a master pattern which isprojected on an LSI substrate through a reducing optical system. Inthese techniques, however, when a high voltage mercury lamp is employedas a light source, the minimum linewidth is about 0.5 μm. For thepurpose of obtaining a pattern size of less than 0.5 μm, there have beendeveloped a direct writing technique using a KrF excimer laser or anelectron beam, and an X-ray unity magnification lithography technique.However, from the viewpoint of mass productivity, process versatilityand so on, great expectation has been directed to the photolithographytechnique.

Under such circumstance, various studies have been made in thephotolithography technique. For a light source, g ray, i ray, X ray,excimer laser, etc. have been studied. For a resist, possibility ofdevelopment of new resist materials and new resist processing such asREL have been studied. Further SREP.CEL image reverse technique has beenstudied.

With respect to the mask fabrication technique, sufficient studies hadnot been made until Levenson, et al. of IBM Corporation have proposed aphase shift method in 1982. Since then, attention has been directed tothe phase shift method. In the phase shift method, the phase of a beampassing through a mask is controlled for the purpose of enhancing theresolution and contrast of a projected image.

The principle of this method will be explained by referring to FIG.20(a)-(d). As shown in FIG. 20(a), a mask comprises a mask pattern 12 oflaminated films made of chrome (Cr) or chrome oxide (Cr₂ O₃) formed on aquartz substrate 11 by a sputtering or the like process, and atransparent film 13 formed on one of an adjacent pair of lighttransmissible areas in the mask pattern 12. The phase of light passingthrough the transparent film area is inverted as shown in FIG. 20(b) andlight passed through the adjacent transmissible areas are canceled eachother at the boundary part between the two transmissible areas as shownin FIG. 20(c). As a result, light intensity at the boundary part betweenthe two light transmissible areas becomes zero and thus the two lighttransmissible areas can be formed in separate patterns on the wafer asshown in FIG. 20(d). In this method, a pattern of 0.7 μm can be resolvedby using a g-ray stepper having a numerical aperture (NA) of 0.28, thusimproving the resolution by about 40%. In this method, in order toinverse the phase of the light passing through the transparent film 13,the transparent film 13 as a phase shifter must satisfy a relationshipof d=λ/2(n-1), where `d` denotes the thickness of the film, `n` denotesthe refractive index of the material of the phase shifter, and `λ`denotes the wavelength of light to be radiated.

Terazawa et al. of Hitachi Ltd. have further developed the method basedon Levenson et al. and proposed a new technique in which the Levensontechnique is applied to an isolated pattern, which principle is shown inFIG. 21(a)-(d). In the Terazawa technique, an isolated pattern `a` andan auxiliary pattern `b` as a dummy which is not resolved by itself areprovided, and phase shifters 13 for inverting the light passingtherethrough are provided to the patterns. In this technique, anisolated space of 0.3 μand a contact hole having a diameter of 0.4 μareresolved in an i-ray stepper, having a numerical aperture (NA) of 0.42.Thus, the resolution is improved by about 30% compared with that of theprior art technique. However, with respect to the contact hole, as thepattern sizes become smaller, the difference in light intensity betweenthe isolated and auxiliary patterns becomes smaller. Thus, thistechnique has resolution limit because the intensity of light passingthrough the transparent areas is weakened as the pattern size becomessmaller.

The aforementioned prior art technique has additional problems that,since the phase shifter is located for every other light transmissiblearea in a line-and-space pattern while a mask layer for the auxiliarypattern is processed such that the phase shifter is provided for theisolated pattern, the position alignment of the shifters to the maskpattern and a selective processing technique become necessary and thenumber of necessary steps will be greatly increased and thus the maskfabrication steps becomes complicated.

In view of such problems, Nitayama, et al. of Toshiba Corporation havesuggested a phase shift mask structure in which a phase shifter isprovided at the periphery of a light transmissible area or a lightblocking area. The principle of this structure is shown in FIG.22(a)-(b). As shown in FIG. 22(a), a mask comprises a mask pattern 12 ofchrome (Cr) and chrome oxide (Cr₂ O₃) formed on a quartz substrate 11,and a transparent film of a phase shifter 13 formed as protruded fromthe periphery of the mask pattern 12. When such a mask is used, thephases of lights passing through the shifters are inverted so that theamplitudes of the lights with phases 0 degree and 100 degree cancel eachother at both ends of the light transmissible area as shown in FIG.22(b). Thus, the light intensity becomes small and the contrast isimproved as shown in FIG. 22(c). As a result, the light intensities atthe both ends of the light transmissible area become substantially zeroand thus the two light transmissible areas can separate the patternformed on the wafer as shown in FIG. 22(d).

The above mask is formed in the following manner. First, as shown inFIG. 23(a), a laminated film 12 of about 100 nm of chrome (Cr) andchrome oxide (Cr₂ O₃) is deposited on the quartz substrate 11 bysputtering or like processes. A resist coated on the film 12 issubjected to electron beam writing operation, and then developed so asto form a resist pattern R.

Next, as shown in FIG. 23(b), the resultant substrate is subjected to anetching operation by wet etching or by reactive ion etching process withuse of the resist pattern as a mask to remove the resist pattern R andform the mask layer 12. Thereafter, as shown in FIG. 23(c), theresultant substrate is coated with a PMMA film, and is exposed withlight from the back face to form a latent image 13S. Then the resultantsubstrate is subjected to a developing operation to form phase shifters13 of a PMMA film pattern as shown in FIG. 23(d). Finally, as shown inFIG. 23(e), the mask layer 12 is subjected to a side etching with use ofthe phase shifters 13 of the PMMA film pattern as a mask, whereby such amask that the phase shifter 13 is protruded from the associated masklayer 12 is fabricated.

In this mask, the PMMA acts as the phase shifter. Since it has a hightransmissivity ratio and a sharp resist profile, the PMMA is anexcellent phase shifter. According to this method, the pattern of thephase shifter can be formed on a self alignment basis. Therefore, themask alignment and selective processing steps are not required and thusa mask can be formed relatively easily.

With the mask, the phase of the light passed through the respectiveshifters is inverted as shown by a broken line in FIG. 22(c), the lightintensity behind the mask layer is remarkably reduced so that intensitydistribution of the whole light is as shown by a solid line in FIG.22(d). This mask has a resolution as fine as nearly half that of theconventional mask.

Assuming that the transmissivity ratio of the phase shifter in the maskis 100%, the width of the shifter is optimized. It is found that theoptimum shift width causing the greatest contrast of image variesdepending on the pattern size. For example, as shown in FIG. 24, when anexcimer stepper having a numerical aperture NA of 0.42 is employed, theoptimum shift width is 0.04 μm (on the wafer) with respect to 0.3 μm ofline-and-space pattern while the optimum shift width is 0.06 μm (on thewafer) with respect to 0.25 μm of line-and-space pattern. In thismanner, the optimum shift which realizes the maximum contrast isdetermined according to the pattern size. The mask having the optimumshift width enables to resolve a fine pattern that can not be resolvedin the conventional technique.

However, in fabricating the above mask, it is necessary to provide afine shifter pattern on the fine pattern, which requires a verydifficult work of controlling the shifter width.

Further, the shifter must be made of either a light transmissible filmor a light blocking film. Generally, a mask used in the VLSI fabricationmust be frequently cleaned because the mask must be used completely freeof dust. For this reason, the phase shift mask must be strong enough towithstand the repetitive cleaning. The shifter formed with resist,however, is not strong enough to withstand the frequent cleaning.

As described above, reticles used in the conventional phase shifttechnique is defective in that it is very difficult to provide the fineshifter pattern on the fine pattern while controlling the shifter widthand making alignment. In addition, the mask is not strong enough towithstand the VLSI fabrication processes.

Further, in order to make the most of the effect of the phase shifting,it is important to optimize a phase difference between light passedthrough the transparent film and light passed through a lightsemi-transparent film as well as an amplitude transmissivity ratiotherebetween. The phase difference and the amplitude transmissivityratio are uniquely determined by the optical constant (complexrefractive index n-ik, where i is unit imaginary number) of these films.In other words, in order to obtain a desired phase difference and adesired amplitude transmissivity ratio, a certain relationship must besatisfied between the optical constant and the film thickness. However,because the optical constant is inherent in the substance, it isdifficult to satisfy the desired relationship with use of a single layerfilm.

FIG. 30 shows a structure of a conventional ideal half-tone phase film.A mask formed according to the conventional method comprises a lighttransmissible film 301 and a semi-transparent film. The semi-transparentfilm is formed to have an amplitude transmissivity ratio of10 to 40%with respect to the light transmissible film and to change the phase oflight passing through the semi-transparent film by 180 degrees withrespect to the light transmissible film. For the purpose of satisfyingthese conditions, the semi-transparent film has a two-layer structurewhich comprises a first layer 302 for adjusting the amplitudetransmissivity ratio and a second layer 303 for adjusting the totalphase difference of the first and second layers 302 and 303 to be 180degrees.

In the conventional half-tone phase shift method, the half-tone film hasa two-layer structure comprising the first layer for adjusting theamplitude transmissivity ratio and the second layers for adjusting thetotal phase difference generated through the first and second layers tobe 180 degrees. However, the two-layer structure is difficult tofabricate, since it requires pattern transfer and the first and secondlayers must be processed in the same sizes.

Especially, it is extremely difficult to cure a defect when the lowerlayer 302 becomes defective as shown in FIG. 31.

SUMMARY OF THE INVENTION

In view of the above circumstances, it is an object of the presentinvention to provide a reticle capable of improving the resolution limitof an exposure system and can realize faithful pattern transfer with aconstant light quantity, and a method for fabricating the reticle.

Another object of the present invention is to provide a reticle capableof satisfying conditions of amplitude transmissivity ratio and phasedifference with use of a single layer film which can exhibit its maximumphase shift effect.

In a reticle in accordance with a first embodiment of the presentinvention, a semi-transparent film pattern is employed in place of alight blocking film pattern for A mask pattern having a size within acertain range. Preferably, the amplitude transmissivity ratio of thesemi-transparent film pattern is adjusted according to the size of themask pattern.

Further, the semi-transparent film pattern preferably comprises aplurality of divided fine regions which are not resolved withlithographic light and have different amplitude transmissivity ratios.

Preferably, the semi-transparent film pattern is formed so that theamplitude transmissivity ratio of the film pattern is adjusted accordingto the occupied area ratio of the fine regions.

Further, the semi-transparent film pattern is preferably formed toinclude a region whose amplitude transmissivity ratio is changed by theion implantation.

A reticle in accordance with a second embodiment of the presentinvention has a laminated layer structure which comprises a phase shiftfilm for changing an optical path, a mask substrate formed on the top orbottom of the phase shift film, and a semi-transparent film having apredetermined transmissivity ratio serving as a transmissivity ratioadjustment layer.

In accordance with a third embodiment of the present invention, theamplitude transmissivity ratio of the material of a phase shifter isadjusted so that a shifter width which affects the contrast can beincreased and the accuracy for a shifter width can be loosened. That is,when a value of a pattern size of the mask pattern divided by a lightexposure condition λ/NA is in a range between 0.34 and 0.68, theamplitude transmissivity ratio of the phase shifter is set to be lessthan 100% and a shifter width is set to be as large as possible to suchan extent that the shifter width does not affect the adjacent pattern.Preferably, the shifter width is set to be sufficiently large and theamplitude transmissivity ratio of the phase shifter is selected toprovide its maximum contrast according to the pattern size.

By using simulation, the distribution of the image intensity projectedon the wafer is estimated with respect to the transmissivity ratio ofthe phase shifter. As a result, it has been found that, when asemi-transparent film pattern having a predetermined transmissivityratio is used in place of a light blocking film pattern, the contrastcan be improved.

According to the simulation result, the present invention has been made.With the arrangement as described above, the resolution to a finepattern image is improved. Further, since this arrangement enables thesimultaneous patterning of the phase shifter and the light blockingfilm, the control for the patterning is easy to perform.

Preferably, the amplitude transmissivity ratio of the semi-transparentfilm pattern is adjusted according to the size of the mask pattern.Thus, the resolution can be further improved.

Further, by arranging a plurality of semi-transparent films havingdifferent transmissivity ratios in a pattern having a resolution belowthat of the lithographic optical system and by determined the occupiedarea ratio of these semi-transparent films to be the optimum amplitudetransmissivity ratio according to the pattern size, a pattern having anamplitude transmissivity ratio according to the size of the pattern canbe easily formed.

As described above, the reticle in accordance with the second embodimentof the present invention has a laminated layer structure which comprisesa phase shift film for providing a different optical path to exposurelight and a transmissivity ratio adjustment layer formed on the top orbottom of the phase shift film and having a predetermined transmissivityratio to the exposure light. With this construction of the reticle, thephase shift film and the transmissivity ratio adjusting film can beselected independently of each other and also can be easily formed. Inaddition, the phase shift film may be made of a silicon di-oxide film, aspin-on glass film or any materials suitably selected from materialsother than resins, and therefore a pattern having a strength enough towithstand repetitive cleaning which take place during the fabrication ofVLSI.

Generally, an optical image depends on NA (numeral aperture of theoptical system) and a wavelength λ. When a pattern size `w` isstandardized as satisfying the relationship:

    y=w/(λ/NA),

the optical image having the same standard size γ becomes completelyanalogous with analogous ratio λ/NA. A factor NA/λ represents a cut-offfrequency in a spatial frequency region and its reciprocal λ/NAcorresponds to frequency for one division when the cut-off frequency isconsidered "1" and is divided by NA/λ. In this manner, by dividing eachpattern size by λ/NA, the position of the size in the spatial frequencyregion can be standardized.

As a result of various experiments conducted with use of the standardsize, it has been found that this is effective, in particular, for maskpatterns having standard size (which are the values divided by λ/NA onthe light exposure condition) of 0.61 or less. Accordingly, by forming amask pattern with a semi-transparent film having a transmissivity ratioof 0-50% and providing a phase shift of 180 degrees for only the maskpattern having standard size of 0.61 or less, a desirable phase shifteffect can be obtained and a pattern, which is impossible to resolve inconventional masks, can be resolved. With respect to the transmissivityratio of the semi-transparent film, it is selected according to thepattern size so that the contrast become optimum.

For the purpose of obtaining the optimum relationship between amplitudetransmissivity ratio and shifter width in an equi-spaced line-and-spacepattern, the inventors of the present application have conductedsimulation researches with use of a program for obtaining light imageintensity distributions. As a result of the simulation, it has beenfound that the amplitude transmissivity ratio of the phase shiftercausing the maximum contrast varies depending on the shifter width, andthe optimum shifter width is increased as the amplitude transmissivityratio decreases. Also, it has been found that this phenomenon takesplace commonly when the value of the pattern size of the mask patterndivided by λ/NA on the light exposure condition is in a range of from0.34 to 0.68. Accordingly, by appropriately adjusting the transmissivityratio of the phase shifter for each pattern size within this range,contrast can be greatly improved effect with the shifter width in arange allowing sufficient accuracy in phase shifting processing.Further, by determining the shifter width to be sufficiently large andby selecting the amplitude transmissivity ratio of the phase shiftersuch that the contrast becomes maximum according to the pattern size,the mask can be fabricated relatively easily.

A reticle in accordance with a fourth embodiment of the presentinvention includes, as a mask pattern, a semi-transparent film patternwhich is made of silicon, a silicon compound or a mixture includingsilicon or made of germanium, a germanium compound or a mixtureincluding germanium and which optical path is different from atransparent part by a predetermined amount with respect to lithographiclight.

Preferably, the semi-transparent film pattern desirably includes an ionimplanted region. Further, the semi-transparent film pattern contains aregion which has a crystalline state changed through heat treatment.

In accordance with a fifth embodiment of the present invention, there isformed on a transparent substrate a semi-transparent film pattern whichis made of silicon, a silicon compound or a mixture including silicon ormade of germanium, a germanium compound or a mixture including germaniumand which has a controlled composition ratio.

Preferably, silicon is used as a target, a predetermined amount ofnitrogen gas is added in a sputtering atmosphere and a nitrogencomposition ratio is controlled to form a nitrogen silicon film and thusa semi-transparent film pattern while adjusting an amplitudetransmissivity ratio.

Further, a silicon di-oxide film is preferably deposited by a CVD methodwhile adjusting the amount of oxygen in a raw material gas to control anoxygen composition ratio, to adjust an amplitude transmissivity ratioand to form a semi-transparent film pattern. Alternatively, a siliconnitride film is deposited by the CVD method while adjusting the quantityof ammonium in the raw material gas to control a nitrogen compositionratio, to adjust an amplitude transmissivity ratio and to form asemi-transparent film pattern.

Further, the fabrication steps preferably include a step for performingan additional ion implantation over the surface of the formedsemi-transparent film pattern and a modification step for finelyadjusting an amplitude transmissivity ratio by changing the crystallinestate through a heat treatment step.

As a result of examination of an optical image intensity distributionprojected on a wafer by changing a shifter transmissivity ratio throughsimulation, it has been found that, when a semi-transparent film patternhaving a predetermined transmissivity ratio is used in place of a lightblocking film pattern, its contrast can be improved.

In view of this respect, the prevent invention has been made and byemploying the above-described arrangement, a fine pattern is easilyresolved. Further, since patterning of the phase shifter is carried outtogether with the light blocking film pattern at a time, easy patterncontrol can be realized.

When it is desired to make the semi-transparent film in the form of asingle layer, it becomes necessary to control the phase of light passingthrough the semi-transparent film to be within a range of 180 degrees±10% with respect to the phase of light passing through the transparentfilm and also to set the transmissivity ratio of the semi-transparentfilm at a desired value.

In order to obtain a maximum resolution with use of a phase shift maskof the semi-transparent film, the optical constant of thesemi-transparent film must satisfy the following conditions (1) and (2).

    E1=t1×EO                                             (1)

    E2=t2×EO                                             (2)

where EO denotes a complex electric field vector for incident light, E1denotes a complex electric field vector for light passed through thetransparent film region, E2 denotes a complex electric field vector forlight passed through the semi-transparent film region, and t1 and t2denote amplitude transmissivity ratio, respectively. In order to obtaina maximum effect for such a phase shift mask as shown in FIG. 2, thefollowing relationship (3) and (4) must be satisfied between thetransmission light in the amplitude transmissivity ratio and phasedifference.

    0.1≦|E1|/|≦0.2    (3)

    170 degrees≦|δ1-δ2|≦190 degrees(4)

where,

E1=|E1|exp(δ1)

E2=|E2|exp(δ2)

The amplitude transmissivity ratios t1 and t2 for light passed throughthe semi-transparent film region and transparent film region areexpressed by functions of air optical constant and the optical constantsand thicknesses of the light transmissible substrate andsemi-transparent film respectively. Shown in FIG. 25 is a range in whichan optical constant (n,k) is satisfied when lithographic light is of ani ray and to meet the relationship (3) and (4). In other words, in orderto fully exhibit the effect of the phase shift method, the opticalconstant of the semi-transparent film must be within an area defined bytwo curves a and b in FIG. 25.

The inventors of the present application have studied materialssatisfying aforementioned conditions and found that the materialsatisfying the aforementioned two conditions is made of one or more ofsilicon, a compound including silicon, a mixture including silicon,germanium, a compound including germanium and a mixture includinggermanium. In particular, it is highly effective that a g-ray region ismade of silicon and an i-ray KrF region as a semi-transparent film ismade of SiN. Their performances are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                                             Amplitude                                                                     Trans-                                                     Wavelength Thickness                                                                             missivity                                       Formation  of Light   of Film to SiO.sub.2                             Material                                                                             of Film    (nm)       (nm)    (%)                                      ______________________________________                                        Si     Sputtering 436        61      17.4                                            (Target:Si)                                                            SiN    Sputtering 365        446     20.0                                            (Target:SiN)                                                           SiN    N.sub.2 Flow                                                                             365        76      15.0                                            Sputtering                                                                    (Target:Si)                                                            SiN    Sputtering 248        267     17.0                                            (Target:SiN)                                                           SiN    N.sub.2 Flow                                                                             248        80      15.0                                            Sputtering                                                                    (Target:Si)                                                            Ge     Sputtering 436        70.3    10.8                                            (Target:Ge)                                                            ______________________________________                                    

By implanting such ions as As, P or B into transparent andsemi-transparent films, the properties of the formed film, e.g., itsoptical constant can be adjusted to some extent.

Further, when the silicon film is heated to over 200° C., the siliconstate can be changed continuously or intermittently from an amorphousstate to a polycrystalline state or from the polycrystalline state to asingle crystalline state to thereby obtain desired physical properties.

In accordance with the second embodiment of the present invention, thedesired values can be obtained by controlling the composition ratio.Since the optical constant is inherent in the material, however, it isimpossible to set the optical constant at an arbitrary value. Thus, theoptimization of the optical constant can be realized, for example, bychanging the composition ratio of a compound.

When the aforementioned conditions are satisfied, the desired phasedifference and transmissivity ratio can be obtained with use of a singlesemi-transparent film.

For example, when silicon is used as a target and a semi-transparentfilm is formed in a mixture gas of argon and nitrogen by a spatteringtechnique, relationship between the quantity of nitrogen gas and theoptical constant is measured and such a variation curve D as shown inFIG. 25 is obtained. Reference symbol c denotes an optimum relationshipcurve and an allowable region c defined by the curves a and b and havinga curve c disposed therebetween satisfies the relationship (3) and (4).An intersection of the curves c and D corresponds to the optimum value.As will be seen from the drawing, when the quantity of nitrogen ischanged to adjust a refractive index and an amplitude transmissivityratio and to form a silicon nitride film, a desired semi-transparentfilm pattern can be obtained. The obtained optimum conditions are thatn=3.30 and k=1.19 when the quantity of nitrogen gas flow at the time ofthe sputtering is 15% and when the film thickness is set to be 83.5 nm,the amplitude transmissivity ratio is 0.1452 and the phase difference is180 degrees, where lithographic light wavelength is 365 nm.

FIGS. 26 and 27 show optical constant ranges optimum forsemi-transparent single-layer films when a KrF excimer laser beam(wavelength: 248 nm) and a g-beam (wavelength: 436 nm) are employedrespectively.

Further, when a silicon nitride film is deposited by the CVD methodwhile adjusting the quantity of ammonium in a raw material gas tocontrol a nitrogen composition ratio or when a silicon di-oxide film isdeposited by the CVD method while adjusting the quantity of oxygen in araw material gas to control an oxygen composition ratio, the optimumamplitude transmissivity ratio and phase difference can be obtained asin the case of forming a semi-transparent film pattern by adjusting theamplitude transmissivity ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a reticle in accordance with a first embodiment of thepresent invention; FIGS. 2(a), 2(b) and 2(c) are resultant lightintensity distributions obtained when simulation is carried out bychanging the transmissivity ratio of the reticle;

FIG. 3 is a graph showing relationship between the variation oftransmissivity ratio and the variation of contrast with respect todifferent standard sizes determined by λ/NA;

FIGS. 4(a) and 4(b) show a second embodiment of the present invention;

FIGS. 5(a) through 5(g) show steps of fabricating the reticle;

FIGS. 6(a) and 6(b) show relationship between implanted-ion dose andtransmissivity ratio;

FIGS. 7(a) through 7(d) show steps of fabricating a reticle inaccordance with a third embodiment of the present invention;

FIGS. 8(a) through 8(d) show steps of fabricating a reticle inaccordance with a fourth embodiment of the present invention;

FIG. 9 shows a fifth embodiment of the present invention;

FIG. 10 shows a reticle in accordance with a sixth embodiment of thepresent invention;

FIGS. 11(a) through 11(c) show steps of fabricating the reticle;

FIGS. 12(a) through 12(d) show steps of fabricating a reticle inaccordance with a seventh embodiment of the present invention;

FIGS. 13(a) and 13(b) are graphs showing relationship between shifterwidth and contrast in the embodiment;

FIG. 14 shows a reticle in accordance with an eighth embodiment of thepresent invention;

FIGS. 15(a) through 15(e) are graphs showing relationship between lightintensity distribution and contrast with respect to different shifteramplitude transmissivity ratios and shifter widths;

FIG. 16 is a graph showing relationship between the shifter amplitudetransmissivity ratio and the contrast of light intensity distributionwith respect to different shift widths;

FIG. 17 shows a relationship between the shifter amplitudetransmissivity ratio and the shifter width;

FIG. 18 is graph showing relationship between light intensitydistribution and contrast with respect to different shifter amplitudetransmissivity ratios and shifter widths;

FIG. 19 shows a relationship between the shifter amplitudetransmissivity ratio and the shifter width;

FIG. 20 (a)-(d) show diagrams for explaining a prior art phase shifttechnique;

FIG. 21 (a)-(d) show diagrams for explaining a second prior art phaseshift technique;

FIG. 22 (a)-(d) show diagrams for explaining a third prior art phaseshift technique;

FIGS. 23(a) through 23(e) show diagrams for explaining a fourth priorart phase shift technique;

FIG. 24 shows relationship between the shifter width and contrast withrespect to different pattern sizes;

FIG. 25 shows a range of optical constant to be satisfied when asemi-transparent film pattern is made in the form of a single layer filmas well as actually measured values of the optical constant (at awavelength of 365 nm);

FIG. 26 shows a range of optical constant to be satisfied when asemi-transparent film pattern is made in the form of a single layer filmas well as actually measured values of the optical constant (at awavelength of 248 nm);

FIG. 27 shows a range of optical constant to be satisfied when asemi-transparent film pattern is made in the form of a single layer filmas well as actually measured values of the optical constant (at awavelength of 436 nm);

FIGS. 28(a)through 28(e) show steps of fabricating a reticle inaccordance with a ninth embodiment of the present invention;

FIG. 29 shows steps of fabricating a reticle in accordance with a tenthembodiment of the present invention;

FIG. 30 shows a conventional phase shifter; and

FIG. 31 shows a conventional phase shifter having a defect therein.

FIG. 32 shows an algorithm for calculating optical constants of asemi-transparent film in which refractive index is an arbitrary value;

FIG. 33 shows an algorithm for calculating optical constants of asemi-transparent film in which extinction coefficient is an arbitraryvalue; and

FIG. 34 shows an algorithm for calculating optical constants of asemi-transparent film in which film thickness is an arbitrary value.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention will be explained in detail withreference to the accompanying drawings.

Embodiment 1:

A reticle in accordance with a first embodiment of the present inventionis shown in cross section in FIG. 1. The illustrated reticle, which is a5-multiple mask, comprises a transparent quartz substrate 1, and a maskpattern 2 made of a semi-transparent film having a film thickness of0.25 μm and a transmissivity ratio of 6% and having a line-and-space(minimum linewidth) of 1.5 μm. The minimum linewidth is transferred onthe wafer to form a line-and-space pattern having a minimum linewidth of0.34 μm.

The semi-transparent film is formed by mixing p-TERPHENYL and PMMA at amixture rate of 1:4, solving the mixture into an ethyl cellosolveacetate solution, rotatably coating the mixture solution to form a filmof 0.25 μm thick on the substrate, and then developing the film to forma pattern having a line-and-space of 1.5 μm.

The reticle thus formed is mounted to a KrF excimer laser stepper havinga projection lens with a numerical aperture NA of 0.42, a wafer coatedwith negative resist known as SAL601 is subjected to a pattern transfer(λk=248 nm and coherency σ=0.5) and then to a development with use of acustom developing solution. As a result, a resist pattern having anaccurate line-and-space (minimum linewidth) of 0.3 μm (which has notbeen obtained in the prior art reticle) can be obtained.

Subsequently, in order to find the optimum shifter transmissivity ratioin the equi-spaced line-and-space pattern, simulation is carried outwith use of an originally prepared program for finding a light imageintensity distribution, which result is shown in FIG. 2. As lightexposure conditions, a beam emitted from a KrF excimer laser is used,and settings are made so that NA=0.42, λ=248 nm (λ/NA=0.59) andcoherency λ=0.5. In this simulation, three sorts of masks are used whichhave the mask patterns 2 of semi-transparent films having threedifferent transmissivity ratios T of 0% (chrome), 6% and 20% and alsohaving a minimum linewidth of 0.3 μm, the mask patterns 2 being formedon the transparent quartz substrate 1 in the similar way to in FIG. 1.Such masks are subjected to measurements of variations in light imageintensity distribution with respect to the different transmissivityratios.

Shown in FIGS. 2(a) to 2(c) are resultant light intensity distributionsobtained when the simulation is carried out with the transmissivityratios T of the mask patterns of 0.3 μm in minimum linewidth changed to0% (chrome), 6% and 20% respectively. It will be seen from the resultantlight intensity distributions that valleys or bottoms in the lightintensity distribution become substantially zero when T=6% whilecontrast is best improved when T=0%.

The contrast is calculated in accordance with the following equation.

    C=(I.sub.max -I.sub.min)/(I.sub.max +I.sub.min)

where, I_(max) denotes the light intensity of the crest in the lightintensity distribution waveform, I_(min) denotes the light intensity ofthe bottom in the light intensity distribution waveform, and C denotesthe contrast. FIG. 3 shows variations in the contrast caused byvariations in the transmissivity ratios of patterns having differentsizes determined by specifications with use of λ/NA. Curves a, b, c, d,e, f, g and h show relationship between the transmissivity ratio andcontrast when the pattern sizes are 0.68 (0.40 μm), 0.63 (0.37 μm), 0.61(0.36 μm), 0.59 (0.35 μm), 0.51 (0.3 μ), 0.42 (0.25 μm), 0.39 (0.23 μm)and 0.34 (0.20 μm) respectively.

With respect to the line-and-space, for the patterns having the patternsizes of above 0.37 μm, as the shifter transmissivity ratio increasesthe contrast correspondingly decreases. The contrast becomes maximum atthe transmissivity ratio 16% for the pattern having the pattern size of0.25 μm, the contrast becomes maximum at the transmissivity ratio 6% forthe pattern having the pattern size of 0.3 μm, and the contrast becomesmaximum at the transmissivity ratio 3% for the pattern having thepattern size of 0.35 μm.

From the graph of FIG. 3, it will be appreciated that, when such a lightblocking film as a chrome film is used for patterns having pattern sizesof above 0.36 μm (0.61 in the standard value) while when asemi-transparent shifter having a transmissivity ratio adjusted asvaried from 0% to 20% is used for patterns having pattern sizes of below0.36 μm, resolution of a fine pattern can be realized.

When the results of FIG. 3 are multiplied by λ/NA in the respectivelight exposure conditions, such a semi-transparent film can be obtainedthat has a transmissivity ratio causing the maximum contrast dependingon the pattern size of the light exposure conditions.

Further it will be seen from FIG. 3 that, when such a light blockingfilm as a chrome film is used for patterns having pattern sizes of above0.61 while when a semi-transparent film having a transmissivity ratioadjusted in a range of from 0% to 50% is used for patterns havingpattern sizes of below 0.61, the contrast can be improved. It will bealso appreciated that the optimum transmissivity ratio is 50% for astandard size of 0.39 and adjustment of the transmissivity ratio at avalue larger than 50% causes no effect for a standard size of 0.34smaller than 0.39. The adjustment of the transmissivity ratio may becarried out by adding a pigment to the semi-transparent film.

Embodiment 2:

Explanation will next be made as to a second embodiment of the presentinvention. FIGS. 4(a) and 4(b) shows major parts of a reticle of thesecond embodiment of the present invention.

The reticle of this embodiment is featured in that a plurality of filmshaving different amplitude transmissivity ratios are arranged in apattern of a resolution limit below that of a lithographic opticalsystem, and the amplitude transmissivity ratio is controlled byadjusting the occupied area ratio of the films according to the patternsize so as to obtain the optimum amplitude transmissivity ratio for eachpattern size, thereby to improve the resolution limit.

In the reticle, lithographic light has a wavelength of about 436 nm, thelithographic light passed through a semi-transparent film pattern as aphase shifter is subjected to a phase inversion of 180 degrees and thenadded to the lithographic light not passed through the semi-transparentpattern so that light intensity becomes sharp at pattern boundaries.

Steps of fabricating the reticle will now be explained.

As shown in FIG. 5(a), first of all, a light-transmissible moltencrystal substrate 1 of a square having a side length of 5 inches and of2.4 mm thick is subjected to a sputtering process to deposit thereon asemi-transparent Cr film 3 having a thickness of 0.035 μm.

Subsequently, as shown in FIG. 5(b), the resultant reticle is coatedthereon with resist R and then subjected to a photolithography processusing electron beam (EB) to be patterned.

Thereafter, as shown in FIG. 5(c), the Cr film 3 is subjected to areactive ion etching operation with use of the resist pattern R as amask and with use of a reactive gas containing, as main components, CH₂Cl₂ and O₂ gases to be patterned. The resultant substrate is thensubjected to a dry etching operation with use of a gas containing CF₄ asa main component, whereby the substrate 1 is etched by an amount ofabout 0.42 μm. A region 4 not bitten or etched through the etchingbecomes a phase adjustment region. At this time, an etched amount d₂ inthe substrate 1 meets the following equation.

    (n.sub.1 -1)×d.sub.1 /λ+(n.sub.2 -1)×d.sub.2 /λ=0.5

where n₁ denotes the refractive index of Cr, d₁ denotes the thickness ofthe Cr film, n₂ denotes the refractive index of the crystal substrate,and d₂ denotes the etched amount in the crystal substrate.

Further, as shown in FIG. 5(d), the resultant substrate is immersed intoa mixture solution of sulfuric acid and hydrogen peroxide to selectivelyremove only the resist pattern R.

After this, as shown in FIG. 5(e), a resist pattern R2 is formed on theresultant substrate to process the Cr film 3 into a fine pattern.

As shown in FIG. 5(f), the resultant substrate is subjected to a dryetching with use of the resist pattern R2 as a mask and with use of areactive gas containing, as main components, CH₂ Cl₂ and O₂ gases toobtain a patterned semi-transparent film made of the Cr film 3 and thensubjected to a liquid phase growth process to selectively form a silicondi-oxide film 5 defined by the resist pattern. The thickness of thesilicon di-oxide film 5 is set so that a phase difference between thelithographic light passed through the Cr film 3 and the lithographiclight passed through the silicon di-oxide film 5 becomes zero.

Finally, the resist R2 is removed with use of an suitable organicsolvent or acid to complete a reticle (see FIG. 5(g)).

The phase shifter in the reticle thus- fabricated is arranged so thatthe occupied area ratio between the fine patterns of the Cr film 3 andsilicon di-oxide film 5 is adjusted according to the pattern size toobtain an optimum pattern.

The film thickness is determined on the condition that the lithographiclight for use of the lithographic light exposure of the reticle has awavelength of 436 nm.

The reticle thus fabricated is mounted in a g-ray stepper having aprojection lens with a numerical aperture (NA) of 0.42 so that thenovolac positive resist PR-1024 of 0.5 μm thick coated on a substrate tobe processed is subjected to a lithographic light exposure, whereby aextremely high reproducibility up to a 0.3 μm pattern can be obtained.

In this case, since a desired amplitude transmissivity ratio is obtainedby adjusting occupied area ratio between the fine patterns of thesemi-transparent Cr film 3 and silicon di-oxide film 5, the control ofthe amplitude transmissivity ratio according to the pattern size can befacilitated.

Although the Cr film 3 and the silicon di-oxide film 5 define steps inthe foregoing embodiment, such steps may be removed and the opticalcharacteristics may be further improved when materials having closerefractive indexes, for example, a chrome oxide film and a silicondi-oxide film are combined.

The present invention is not limited to Cr as the material of thesemi-transparent film but any metallic material or other suitablematerial may be employed. That is, any material may be employed bysetting the film thickness to be thin. Further, the invention is notrestricted to silicon di-oxide as the transparent film but suchmaterials as calcium fluoride (CaF), magnesium fluoride (MgF) oraluminum oxide (Al₂ O₃) may be employed.

Embodiment 3:

Detailed explanation will then be made as to a third embodiment of thepresent invention.

The present embodiment, which is based on the fact that transmissivityratio can be gradually changed by ion implantation, is featured in thatfine adjustment of the amplitude transmissivity ratio can be realized.

A transparent substrate of molten crystal is subjected thereon to an ionimplantation with use of silicon ions and an acceleration voltage of 30KeV and then subjected to measurements at a wavelength of 436 nm. Themeasurement result is shown in FIG. 6(a) wherein a horizontal axisrepresents implanted ion dose and a vertical axis representstransmissivity ratio. It will be seen from the drawing that thetransmissivity ratio monotonously decreases as the dose increases.

In the case of this reticle, lithographic light having a wavelength of436 nm is used so that the lithographic light passed through thesemi-transparent film pattern as the phase shifter is inverted by 100degrees and then added to the lithographic light not passed through thesemi-transparent film pattern, whereby light intensity is sharpened atpattern boundaries.

Explanation will next be made as to how to fabricate the reticle.

First, as shown in FIG. 7(a), a light-transmissible, molten crystalsubstrate 1 of 2.4 mm thick and of a square shape each having a sidelength of 5 inches is subjected to a silicon ion implantation with useof a dose of 7.0×10¹⁷ /cm² and an acceleration voltage of 30 KeVuniformly all over the entire surface thereof to form a semi-transparentlayer 6. The resultant semi-transparent layer 6 has a transmissivityratio of 6% with respect to lithographic light having a wavelength of436 nm.

Thereafter, as shown in FIG. 7(b), the resultant substrate is coatedthereon with resist R and then subjected to an EB lithographic processfor patterning.

Next, as shown in FIG. 7(c), the resultant substrate is subjected to areactive ion etching operation with use of the resist pattern R as amask and a reactive gas containing, as a major component, CF₄ gas toform grooves T therein having an etched depth of about 0.47 μm. A regionnot etched through the etching operation corresponds to a phaseadjustment region.

A reticle thus formed is mounted in a g-ray stepper having a projectionlens with an NA of 0.42 and the novolac positive resist PR-1024 of 0.5μm thick coated on a substrate to be processed is subjected tolithographic light, whereby a very high reproducibility up to a 0.3 μmpattern can be obtained.

The semi-transparent layer in the thus-fabricated reticle may be formedso that the dose or acceleration voltage is varied according to thepattern size or writing with use of a focused ion beam (FIB) techniqueis carried out to form a fine ion-implanted region and to obtain anoptimum pattern.

Although silicon ion implantation has been carried out in the foregoingembodiment, the present invention is not limited to only the siliconions but other suitable ions may be suitably employed. When Au ions areimplanted under the same implantation conditions as shown in FIG. 6(a),however, the transmissivity ratio is saturated at 80% as the dose isincreased and a further increase of the dose results in impossibleadjustment as shown in FIG. 6(b). Accordingly, it is necessary to selectsuch ion and implantation energy that the transmissivity ratiomonotonously varies as the dose of implantation ions increases.

Embodiment 4:

A fourth embodiment of the present invention will be detailed. In thepresent embodiment, ion implantation is carried out through a resistpattern to selectively form a semi-transparent layer.

Explanation will then be made as to how to fabricate the reticle.

First, as shown in FIG. 8(a), a light-transmissible, molten crystalsubstrate 1 of 2.4 mm thick and of a square shape each having a sidelength of 5 inches is coated thereon with resist R, subjected to an EBlithography process for patterning, and then subjected to a silicon ionimplantation uniformly all over the surface thereof through the resistpattern R to form a semi-transparent layer 7 as shown in FIG. 8(b). Atthis time, when the resist pattern R in the pattern region is arrangedto be adjusted with respect to each transfer pattern size, thetransmissivity ratio can be controlled to have a desired value for eachregion.

Then selectively formed on the resultant substrate is a silicon di-oxidefilm 8 defined by the resist pattern through a liquid phase growthprocess. In this case, the thickness of the silicon di-oxide film 8 isselected so that a phase difference between the lithographic lightpassed through the semi-transparent layer 7 and the lithographic lightpassed through the transparent substrate becomes 180 degrees (refer toFIG. 8(c)).

Finally, the resist R is removed with use of suitable organic solvent oracid to complete a reticle (Refer to FIG. 8(d)).

The phase shifter in the thus-formed reticle is arranged so that theoccupied area ratio of the fine patterns of a stacked layer structure ofthe silicon di-oxide film 8 and semi-transparent layer 7 is adjustedaccording to the pattern size to obtain an optimum pattern.

Embodiment 5:

A fifth embodiment of the present invention will be detailed below.

FIG. 9 shows the cross-sectional view of a reticle in accordance withthe fifth embodiment of the present invention. The reticle comprises atransparent quartz substrate 21 and mask patterns 22 and 23 formed onone surface of the substrate 21 and having different sizes. Morespecifically, such a light blocking film 22 as a chrome film is used fora pattern having a pattern standard size of 0.61 or more, while thesemi-transparent film 23 is used for a pattern having a pattern standardsize of less than 0.61, the semi-transparent film being set to be in atransmissivity ratio range of 0-50%. In this case, all the patternsformed on the mask have a very good contrast and the resolution is alsoremarkably increased.

Embodiment 6:

A sixth embodiment of the present invention will next be explained indetail.

FIG. 10 shows the cross-sectional view of a reticle in accordance withthe sixth embodiment of the present invention. In the reticle, a chromelight blocking film (not shown) is used for a pattern having a size of0.37 μm or more (0.61 or more in the standard value), while asemi-transparent pattern having a transmissivity ratio adjusted to be ina range of from 0% to 20% is used for a pattern having a size of lessthan 0.37 μm, whereby resolution of a fine pattern can be realized.

That is, the reticle is featured by comprising a transparent quartzsubstrate 31, a semi-transparent film 32 of a thin chrome film formed onthe substrate 31 to have an amplitude transmissivity ratio of 25% (6.25%in light intensity transmissivity ratio), and a transparent silicondi-oxide film 33 as a phase shifter formed on the top of thesemi-transparent film 32. The silicon di-oxide film 33 acts as a phaseshifter to incident exposure light. The exposure light passed throughthe transparent silicon di-oxide film 33 as the phase shifter isinverted by 100 degrees and then added to exposure light not passedthrough the phase shifter 33, so that the added exposure light is sharpin light intensity at the pattern boundaries.

Explanation will next be made as to steps for fabricating the reticle.First, as shown in FIG. 11(a), the thin semi-transparent film 32 of Cris formed on one surface of the light-transmissible quartz substrate 31by a sputtering technique and then the silicon di-oxide film 33 actingas a phase shifter of exposure light is deposited on the top of thesemi-transparent film 32. In the present embodiment, the thickness ofthe silicon di-oxide film 33 as the phase shifter is set to provide aphase shift of 180 degrees.

As shown in FIG. 11(b), the resultant substrate of FIG. 11(a) is coatedwith a resist 34, subjected to a patterning operation by a lithographybased on electron beam (EB) exposure. The silicon di-oxide film 34 isthen subjected to a patterning operation with use of the patternedresist 34 as a mask by means of a reactive ion etching using a reactivegas containing a CF₄ gas as its main component.

As shown in FIG. 11(c), the thin semi-transparent film 32 of Cr materialis subjected to a patterning operation by a dry etching with use of CH₂Cl₂ and O₂ as main components and the resist 34 is removed with use ofan organic solvent or acid, thus completing a reticle.

Since the phase shifter of the reticle thus fabricated is made in theform of patterns of the semi-transparent film 32 and the silicondi-oxide film 33 as an inorganic film, even in the case where the maskis repetitively cleaned in the fabrication steps of a VLSI, the mask hasa strength sufficiently able to withstand the repetitive cleaningoperations and thus is long in operational life and high in reliability.

The thickness of the film is determined in the case where an ultravioletlight having a wavelength of 436 nm emitted from a KrF excimer laser isused as the exposure light of the reticle. In the present embodiment,the thickness of the silicon di-oxide acting as the phase shifter is setto be λ/2(n-1), where λ denotes the wavelength of the exposure light andn denotes the refractive index of the silicon di-oxide film.

When the thus-fabricated reticle is mounted to a g-ray stepper having aprojection lens with a numerical aperture NA of 0.42 so that 0.5 μm ofnovolac positive resist PR-1024 coated on a substrate to be processed issubjected to a light exposure, 0.3 μm of pattern can be reproduced witha very high accuracy.

For comparison, when light exposure is carried out with use of aconventional reticle without the formation of the phase shifter layerand having exactly the same structure as the foregoing embodiment exceptfor the non-formation of the phase shifter layer, its resultantresolution is about 0.4 μm at most. It will also be seen from thesolution comparison that, when the reticle fabricated according to theembodiment of the present invention as well as the photolithographyusing the mask are employed, a highly accurate pattern can be obtained.

The material of the semi-transparent film is not limited to Cr but mayalso be other metallic material or material. That is, any material maybe employed therefor, for example, by setting the film thickness to bethin. Further, the material of the transparent film is not restricted tosilicon di-oxide but such other material as calcium fluoride (CaF),magnesium fluoride (MgF) or aluminum oxide (Al₂ O₃) may be also used.Furthermore, an anti-reflection film made of magnesium fluoride or thelike may be formed on the top or bottom of the pattern by a sputtering.

Embodiment 7:

FIG. 12 shows cross-sectional views of steps for fabricating a reticlein accordance with a seventh embodiment of the present invention. Thefeature of the present reticle is that a Cr film pattern of 100 nm thickis formed on one surface of a light-transmissible quartz substrate 11,on which formed is a phase shifter 121 having a shifter width of 0.6 μmas a semi-transparent film having a film thickness of 0.25 μm and atransmissivity ratio of 30%.

First, as shown in FIG. 12(a), a mixture of p-TERPHENYL and PMMA at amixture rate of 1:4 is solved into an ethyl cellosolve acetate solutionand then rotatably coated on one surface of the light-transmissiblequartz substrate 11 coated thereon with a Cr film pattern of 100 nmthick to thereby form a film of 0.25 μm thick thereon. The phase shifterhas an amplitude transmissivity ratio of 30%.

Next, as shown in FIG. 12(b), the resultant substrate of FIG. 12(a) issubjected to a backside light exposure having a wavelength of 200-300 nmwith use of a mercury lamp. As shown in FIG. 12(c), the substrate ofFIG. 12(b) is then subjected to a developing operation with use of acustom developing solution TSK. Thereafter, as shown in FIG. 12(d), thesubstrate of FIG. 12(c) is subjected to a side etching with use of acerium (II) nitrate ammonium solution so that a shifter width on themask becomes 0.6 μm, whereby a mask is completed.

The thus-fabricated reticle is mounted to the KrF excimer laser stepperhaving a projection lens with a numerical aperture of 0.42 so that awafer having a negative resist known as SAL601 coated on a siliconsubstrate is subjected to a pattern transfer (λ=248 nm and coherencyσ=0.5) and then developed with use of a custom developing solution.

As a result, resist patterns with fine minimum linewidths (orline-and-space) of 0.25 μm and 0.3 μm can be obtained, which can not beobtained in conventional reticles.

When the prior art shifter has an amplitude transmissivity ratio of100%, the optimum shifter width for a pattern size of 0.3 μm is 0.2 μmon the mask, which results in generation of variations of about ±0.1 μm.A reduction in the contrast caused by an error in the shifter width isas fairly large as 0.05 (in relative value) as shown in FIG. 13(a). Whenthe phase shifter has an amplitude transmissivity ratio of 30% and anoptimum shifter width of 0.6 μm, on the other hand, a reduction in thecontrast caused by even an error of ±0.1 μm is as very small as 0.005 asshown in FIG. 13(b).

In this way, by reducing the transmissivity ratio of the shifter toincrease the shifter width, a reduction in the contrast caused by theshifter width error can be suppressed and consequently a good resistpattern can be provided.

Embodiment 8:

Next, for the purpose of obtaining the optimum relationship between theamplitude transmissivity ratio and a shifter width in such anequi-spaced line-and-space pattern as shown in FIG. 14, simulation iscarried out with use of a program for finding a light image intensitydistribution. As the light exposure conditions, a laser beam emittedfrom a KrF excimer laser is used as exposure light, and settings aremade so that NA=0.42, λ=248 nm and coherency σ=0.5. Combinations of theamplitude transmissivity ratio and shifter width producing the highestcontrast when the amplitude transmissivity ratio and shifter width arevaried are shown in FIGS. 15(a) to 15(e). It will be seen from theresults of the drawings that the smaller the amplitude transmissivityratio is the larger the shifter width is.

FIG. 16 shows variations in the contrast of the light image intensitydistribution when the shifter width is fixed and the amplitudetransmissivity ratio is varied in a mask pattern having a line-and-space(minimum linewidth) of 0.3 μm. It will be seen from FIG. 16 that theamplitude transmissivity ratio of the shifter causing the maximumcontrast varies from shifter width to shifter width. Shown in FIG. 17are measurement results showing a relationship between the shifteramplitude transmissivity ratio and the shifter width when the contrastis maximum. It will be appreciated from the drawing that, as the shifteramplitude transmissivity ratio is decreased, the shifter width isincreased.

FIG. 18 shows variations in the contrast of the light image intensitydistribution when the shifter width is fixed and the amplitudetransmissivity ratio is varied in a mask pattern having a line-and-space(minimum linewidth) of 0.25 μm. FIG. 19 shows measurement resultsshowing a relationship between the shifter amplitude transmissivityratio and the shifter width when the contrast is maximum.

As a result of the repetitive tests, it has been found that the abovephenomenon commonly takes place when the pattern size of the masterpattern divided by λ/NA on the light exposure condition falls within arange of 0.34-0.68. Accordingly, by suitably adjusting thetransmissivity ratio of the shifter in the respective pattern sizeswithin the above range, a great contrast improvement effect can berealized with the shifter widths in such a range as to provide asufficient accuracy in the shifter processing.

Although explanation has been made in connection with the foregoingembodiments of the present invention wherein the transparent film as thephase shifter is made of a polymethyl methacrylate film as the resist orof the silicon di-oxide layer as the inorganic film, the presentinvention is not limited to the specific examples but any material maybe used so long as the material exhibit a high transmissivity ratio withrespect to the exposure light having a wavelength of 436 nm or less. Forexample, the inorganic film may be made of calcium fluoride (CaF),magnesium fluoride (MgF), aluminum oxide (Al₂ O₃) or the like material.Further, the phase shifter resist may be polymethyl methacrylate,polytrifluoroethyl-α-chloroacrylate, chloromethylation polystyrene,polydimethyl glutaric imide, polymethyl isopropenyl ketone or the likematerial. The thickness, etc. of the pattern may be suitably modifieddepending on the material and lithography light.

Furthermore, the materials of the light-transmissible substrate andlight-blocking film are also not limited to the specific examples usedin the foregoing embodiments, but may be suitably modified as necessary.In addition, it is not necessary that the phase shifter layer alwaysprovide a phase shift of 180 degrees, and any phase shift somewhatdeviated from 180 degrees may be employed so long as the light intensitydistribution of the pattern edge can be sharply reduced in the vicinityof 100 degrees.

Embodiment 9:

FIG. 28 shows steps of fabricating a reticle in accordance with a ninthembodiment of the present invention. This reticle is featured in that asilicon pattern formed by a sputtering technique is used as asemi-transparent film pattern, and this mask can be used as a reticlefor g-ray projection lithography.

First, as shown in FIG. 28(a), a silicon di-oxide substrate 101 issubjected to a sputtering operation to form a silicon film 102 of 59 nmthick thereon. The silicon film 102 having a thickness of 59 nm has arefractive index n of 4.93 and provides a phase difference of 180degrees with respect to the g ray of a mercury lamp. Its amplitudetransmissivity ratio is 17.4% with respect to the amplitudetransmissivity ratio of the silicon di-oxide substrate 101 as atransparent layer.

Next, as shown in FIG. 28(b), resist for electron beam is deposited onthe resultant substrate in the form of a layer 103 having a thickness of0.5 μm, and further an electrically conductive film 104 is formedthereon to have a thickness of about 0.2 μm.

An electron beam writing is carried out onto the conductive film 104 at3 μC/cm² and then subjected to a developing operation to form a resistpattern 103 (refer to FIG. 28(c)). The purpose of the formation of theconductive film 104 is to prevent the charging up of the electron beamwhen the resist is insulating material.

The resultant substrate is subjected to a chemical dry etching (CDE)with use of the resist pattern 103 as a mask and a mixture gas of CF₄ anO₂ to partly remove the silicon film 102 exposed to the resist pattern(refer to FIG. 28(d)).

Finally, the resist pattern 103 is removed to obtain a silicon filmpattern 102 (FIG. 28(e)).

In this way, a phase shifter in the form of a desired single-layer,semi-transparent film can be obtained.

Although the formation of the silicon film as the phase shifter has beencarried out by means of the sputtering in the present embodiment, a CVDtechnique or the like method may be employed. The silicon film may beset to have a suitable thickness so long as the thickness lies in arange not departing from the subject matter of the present invention.

Further, the processing of the silicon film may be effected by means ofreactive ion etching.

With use of the thus-fabricated reticle, a layer of 1.5 μm thick and ofresist PER7750 (manufactured by Nippon Gousei Gomu K.K.) coated on asubstrate is subjected to a 1/5 reduction lithography with use of g ray(NA=0.54, σ=0.5). At this time, the quantity of lithographic light is300 mJ/cm². When the mask of the present invention is used, a 0.45 μmpattern can be resolved with a focus margin of 0.7 μm, though the samepattern is resolved with a focus margin of 0 μm in the conventionalmask.

With respect to a contact hole pattern, a 0.50 μm pattern is confirmedto be resolved with a focus margin of 1.5 μm in the present invention,though the same pattern is not resolved in the conventional masks.Further, when processing of a substrate is carried out with use of theresist pattern transferred and formed using the above mask, a betterprocessed shape can be obtained.

Embodiment 10:

Detailed explanation will be directed to a tenth embodiment of thepresent invention. A reticle according to the present embodiment isfeatured in that a Ge pattern formed by a sputtering method is used as asemi-transparent film pattern, and can be used as a projection reticlefor g ray.

In this example, a Ge film 102 of 70 nm thick is formed on a silicondi-oxide substrate by a sputtering method. When the Ge film of 7.03 nmthick is used to have a refractive index n of 4.1 and provide a phasedifference of 180 degrees with respect to the g ray of a mercury lamp,an amplitude transmissivity ratio is 10.8% with respect to the amplitudetransmissivity ratio of the silicon di-oxide substrate 101.

As in the embodiment 9, resist for electron beam is deposited on theresultant substrate in the form of a layer having a thickness of 0.5 μmand further an electrically conductive film having a thickness of about0.2 μm is formed thereon.

An electron beam writing is carried out on the conductive film at 3μC/cm² and then developing operation is effected to form a resistpattern.

The resultant substrate is subjected to a chemical dry etching (CDE)process with use of the pattern as a mask and a Cl₂ gas to remove the Gefilm exposed from the resist pattern. Finally, the resist pattern isremoved to obtain a Ge film pattern. In this way, a phase shifter in theform of a desired single-layer semi-transparent film can be obtained.

The formation of the Ge film as the phase shifter has been carried outby means of sputtering in the present embodiment, but a depositionmethod or the like may be employed. Further, the film thickness may beset to have a suitable value so long as the thickness lies in a rangenot departing from the subject matter of the present invention.

Also, the processing of the Ge film may be effected by means of reactiveion etching with use of such a gas containing fluorine as CF₄ or C₂ F₆.

With use of the thus-formed reticle, a layer of 1.5 μm thick and ofresist PER7750 (manufactured by Nippon Gousei Gomu K. K.) coated on asubstrate is subjected to a 1/5 reduction lithography with use of g ray(NA=0.54, σ=0.5). At this time, the quantity of lithographic light is300 mJ/cm². When the mask of the present invention is used, a 0.45 μmpattern can be resolved with a focus margin of 0.7 μm, though the samepattern is resolved with a focus margin of 0 μm in the conventionalmask.

With respect to a contact hole pattern, a 0.50 μm pattern is confirmedto be resolved with a focus margin of 1.5 μm in the present invention,though the same pattern is not resolved in the conventional mask.

Embodiment 11:

Detailed explanation will be directed to an eleventh embodiment of thepresent invention. A reticle according to the present embodiment isfeatured in that an SiN pattern formed by a sputtering method is used asa semi-transparent film pattern, and can be used as a projection reticlefor i ray.

In this example, an SiN film of 446 nm thick is formed on a silicondi-oxide substrate with use of silicon nitride which nitrogen amount isadjusted as a target by a sputtering method. When the SiN film is usedto have a refractive index n of 2.23 with respect to the i ray of amercury lamp, an amplitude transmissivity ratio is 20.1% with respect tothe amplitude transmissivity ratio of the silicon di-oxide substrate.

As in the embodiments 1 and 2, resist for electron beam is deposited onthe resultant substrate in the form of a layer having a thickness of 0.5μm and further an electrically conductive film having a thickness ofabout 0.2 μm is formed thereon. The electron beam writing is carried outon the conductive film at 3 μC/cm² which is then developed to form aresist pattern.

The resultant substrate is subjected to a chemical dry etching (CDE)process with use of the pattern as a mask and a CF₄ gas to remove theSiN film exposed from the resist pattern. Finally, the resist pattern isremoved to obtain an SiN film pattern. In this way, a phase shifter inthe form of a desired single-layer semi-transparent film can beobtained.

The formation of the SiN film as the phase shifter has been carried outby means of sputtering in the present embodiment, but a depositionmethod or the like may be employed. Further, the film thickness may beset to have a suitable value so long as the thickness lies in a rangenot departing from the subject matter of the present invention.

With use of the thus-formed reticle, a layer of 1.5 μm thick and ofresist PER7750 (manufactured by Nippon Gousei Gomu K. K.) coated on asubstrate is subjected to a 1/5 reduction lithography with use of i ray(NA=0.5, σ0.5). At this time, the quantity of lithographic light is 300mJ/cm² When the mask of the present invention is used, a 0.35 μm patterncan be resolved with a focus margin of 0.8 μm, though the same patternis resolved with a focus margin of 0 μm in the conventional mask.

With respect to a contact hole pattern, it is confirmed that a 0.40 μmpattern is resolved with a focus margin of 1.3 μm in the presentinvention, though the same pattern is not resolved in the conventionalmask.

Embodiment 12:

Detailed explanation will be directed to a twelfth embodiment of thepresent invention. A reticle according to the present embodiment isfeatured in that an SiN pattern formed by a sputtering method is used asa semi-transparent film pattern, and can be used as a mask for KrF (248nm).

In this example, an SiN film of 76 nm thick is formed on a silicondi-oxide substrate with use of silicon as a target while adding apredetermined amount of nitrogen gas thereto by a sputtering method.When the SiN film is used to have a refractive index n of 2.68 withrespect to KrF, an amplitude transmissivity ratio is 15% with respect tothe amplitude transmissivity ratio of the silicon di-oxide substrate.

As in the embodiments 9 and 11, resist for electron beam is deposited onthe resultant substrate in the form of a layer having a thickness of 1.5μm and further an electrically conductive film having a thickness ofabout 0.2 μm is formed thereon. The electron beam writing is carried outon the conductive film at 6 μC/cm² which is then developed to form aresist pattern.

The resultant substrate is subjected to a chemical dry etching (CDE)process with use of the pattern as a mask and CF₄ and O₂ gases to removethe SiN film exposed from the resist pattern. Finally, the resistpattern is removed to obtain an SiN film pattern. In this way, a phaseshifter in the form of a desired single-layer semi-transparent film canbe obtained.

The formation of the SiN film as the phase shifter has been carried outby means of sputtering in the present embodiment, but a CVD method withuse of ammonium and a gas containing silane or the like method may beemployed. Further, the film thickness may be set to have a suitablevalue so long as the thickness lies in a range not departing from thesubject matter of the present invention.

With use of the thus-formed reticle, a layer of 1.0 μm thick and of KrFresist XP8843 (manufactured by Shipule company) coated on a substrate issubjected to a 1/5 reduction lithography with use of a KrF excimer laserbeam (NA=0.4, σ=0.5) for patterning. At this time, the quantity oflithographic light is 40 mJ/cm². When the mask of the present inventionis used, a 0.30 μm pattern can be resolved with a focus margin of 0.7μm, though the same pattern is resolved with a focus margin of 0 μm inthe conventional mask.

With respect to a contact hole pattern, it is confirmed that a 0.30 μmpattern is resolved with a focus margin of 1.2 μm in the presentinvention, though the same pattern is not resolved in the conventionalmask.

Embodiment 13:

FIG. 29 shows steps of fabricating a reticle in accordance with athirteenth embodiment of the present invention. This reticle is featuredin that a silicon nitride pattern formed as controlled in its nitrogencomposition ratio by a sputtering technique is used as asemi-transparent film pattern, and this mask can be used as a mask fori-ray projection lithography.

First, as shown in FIG. 29(a), a molten crystal substrate of, e.g., 2.4mm thick and of a square shape each having a side length of 5 inches isused as a light-transmissible substrate, and the substrate is subjectedthereon to a sputtering process with use of silicon as a target in anatmosphere of a mixture gas of nitrogen and argon (15% of nitrogencontent) to form a silicon nitride film 202 of 80 nm thick. When thesilicon nitride film 202 has a refractive index n of 3.4 with respect tothe i ray of the mercury lamp, an amplitude transmissivity ratio of thesemi-transparent film to the molten crystal substrate 201 as atransparent layer is 15%.

Next, as shown in FIG. 29(b), resist is coated on the resultantsubstrate in the form of a resist layer 203 having a thickness of 0.5μm, and further subjected to an EB lithography process to form thereon adesired resist pattern.

Thereafter, the resultant substrate is subjected to a chemical dryetching (CDE) process with use of the resist pattern 203 as a mask and amixture gas of CF₄ and O₂ to remove the silicon nitride film 202 exposedfrom the resist pattern (refer to FIG. 29(c)).

The resultant substrate is immersed into a mixture solution of sulfuricacid and hydrogen peroxide to remove the resist pattern 202 and toobtain a silicon nitride film pattern 202 (refer to FIG. 29(d)).

In this way, a phase shifter made of a desired single-layersemi-transparent film can be obtained.

Although the formation of the silicon nitride film as the phase shifterhas been carried out by means of the sputtering with use of silicon asthe target and while controlling the quantity of nitrogen gas in thepresent embodiment, a sputtering process with use of a mosaic target ofsilicon and silicon nitride or a CVD technique for controlling a gasratio may be employed. Further, the film thickness may be set to have asuitable value which lies in a range not departing the subject matter ofthe present invention.

In addition, the fine adjustment of the refractive index and amplitudetransmissivity ratio may be carried out by means of ion implantationand/or heat treatment for surface modification.

Algorithm for calculating optical constants of the semi-transparent filmwill now be described.

Referring to FIG. 32, initial values for two of the refractive index,extinction coefficient and film thickness of the semi-transparent filmare set with respect to wavelength of an exposure light beam while aninitial value of the remaining one of the refractive index, extinctioncoefficient and film thickness is an arbitrary value.

It is assumed that the refractive index is an arbitrary value and theinitial values of the extinction coefficient k and the film thickness dare to be set.

The initial values of the extinction coefficient k and the filmthickness d are determined such as to satisfy the following equations(step 1001).

For example,

    d=λ/(2(n-1)), k=-(λ/2πd) log t

where λ:

wavelength of exposure light

n: refractive index of half-tone film at wavelength λ

k: extinction coefficient of half-tone film at wavelength λ

φ: phase difference

t: amplitude transmissivity ratio

Then, the phase difference φ and transmissivity ratio t are calculatedwhile considering multiple reflection (step 1002) and the results of thecalculation are represented by the phase difference φ_(c) and thetransmissivity ratio respectively (step 1003).

These calculated values are compared with ideal values (step 1004), andit is determined whether errors between the calculated value and theideal values are within allowable ranges.

When the errors are outside of the allowable ranges, the extinctioncoefficient k and film thickness d are reset (step 1005) and theoperation goes back to step 1002 where the phase difference φ andtransmissivity ratio t are calculated while considering multiplereflection. Then, a loop of steps 1005, 1002, 1003 and 1004 arerepeated.

When the errors are within the allowable ranges, the calculated valuesof the extinction coefficient k and the film thickness t are determinedas ideal values thereof (step 1006).

Thus, the optical constants of a half-tone film having idealtransmissivity ratio and phase difference can be easily determined.

The values satisfying the above equations are not necessarily used asthe initial values of the extinction coefficient k and the filmthickness d. However, by using these values, the convergence of therepeated operation in the loop will be faster.

Further, the loop (step 1005) is not necessarily that in FIG. 32. Anyequations may be used as long as they feed back the difference betweenthe calculated values and the ideal values.

FIG. 33 shows an algorithm in which the extinction coefficient is anarbitrary value and FIG. 34 shows an algorithm in which the filmthickness is an arbitrary value.

As has been disclosed in the foregoing, in accordance with the reticleof the present invention, the light blocking film pattern can bereplaced by the semi-transparent film pattern, so that the contrast canbe improved and the resolution can be enhanced.

Since the reticle in accordance with the second embodiment of thepresent invention is of a stacked layer structure which comprises thephase shift film for providing a different optical path to the exposurelight, the semi-transparent film as the transmissivity ratio adjustmentlayer formed on the top or bottom of the phase shift film and having apredetermined transmissivity ratio to the exposure light, the film forshifting the phase of the light and the film for adjusting thetransmissivity ratio can be selected independently of each other andthus the reticle high in strength against its cleaning operations.

In accordance with the third embodiment of the present invention, theamplitude transmissivity ratio of the material of the phase shifter isadjusted so that the shifter width for effectively improving thecontrast can be made large and the accuracy necessary for the shifterwidth can be loosened, whereby the mask having a high accuracy and ahigh contrast can be easily fabricated.

In this way, in accordance with the reticle of the present invention,the faithful pattern having a high accuracy can be fabricated withoutthe dependency of the pattern density.

Further, in accordance with the present invention, since the reticle canbe made in the form of a single-layer film having an optimum opticalconstant, there can be realized a reticle which is high in reliabilityand easy in the control of the amplitude transmissivity ratio and phasedifference.

What is claimed is:
 1. A method of exposing light comprising the stepsof:forming a mask pattern made of a mask film pattern formed on alight-transmissible substrate, the mask pattern being a semi-transparentfilm which causes a length of an optical path for a light beam passingtherethrough to be different from a length of an optical path for alight beam passing through the substrate by a predetermined amount, thestep of forming a mask pattern comprising the steps of:determiningranges of refractive index n and extinction coefficient k of thesemi-transparent film within which an amplitude transmissivity ratio Tand a phase difference φ between the semi-transparent film and air thathas the same thickness of the semi-transparent film fall within rangesnecessary to obtain desired resolution and depth of focus, obtainingrelationship in the form of a curved line between the refractive index nand the extinction coefficient k by changing conditions for formation ofthe mask pattern, selecting values of the refractive index and theextinction coefficient k from the ranges thus determined, anddetermining the conditions for formation of the mask pattern such thatthe refractive index n and the extinction coefficient k become theselected values, and forming a mask film pattern based on the conditionsfor formation of the mask pattern; and irradiating an exposure beam ontoa resist through a reticle using the mask pattern.
 2. A method ofexposing light as set forth in claim 1, wherein the step of forming themask film pattern comprises the step of forming a silicon nitride orsilicon oxide film having desired values of amplitude transmissivityratio and phase difference by mixing a predetermined quantity ofnitrogen or oxygen gas in a sputtering atmosphere using silicon as atarget and controlling a nitrogen or oxygen composition ratio based onthe conditions for formation of the mask pattern.
 3. A method ofexposing light as set forth in claim 2, wherein said phase difference issubstantially π.
 4. A method of exposing light as set forth in claim 1,wherein said exposure beam is a g ray and the step of forming the maskfilm pattern comprises the step of forming a mask pattern made ofamorphous silicon based on the conditions for formation of the maskpattern, thereby forming a mask pattern having desired values ofamplitude transmissivity ratio and phase difference.
 5. A method ofexposing light as set forth in claim 4, wherein, said phase differenceis substantially π.
 6. A method of exposing light as set forth in claim4, wherein the step of forming the mask film pattern comprises the stepof forming a silicon nitride or silicon oxide film having desired valuesof amplitude transmissivity ratio and phase difference by mixing apredetermined quantity of nitrogen or oxygen gas in a sputteringatmosphere using silicon as a target and controlling a nitrogen oroxygen composition ratio based on the conditions for formation of themask pattern.
 7. A method of exposing light as set forth in claim 1,wherein said exposure beam is an i ray.
 8. A method of exposing light asset forth in claim 1, wherein said exposure beam is an g ray.
 9. Amethod of exposing light as set forth in claim 1, wherein the step offorming the mask film pattern comprises the step of depositing a silicondioxide film while adjusting the quantity of oxygen in a raw materialgas by using a CVD technique based on the conditions for formation ofthe mask pattern so as to control an oxygen composition ratio in thesilicon dioxide film, thereby forming a mask pattern having desiredvalues of amplitude transmissivity ratio and phase difference.
 10. Amethod of exposing light as set forth in claim 1, wherein the step offorming the mask film pattern comprises the step of depositing a siliconnitride film while adjusting the quantity of ammonium in a raw materialgas by a CVD technique based on the conditions for formation of the maskpattern so as to control a nitrogen composition ratio, thereby forming amask pattern having desired values of the amplitude transmissivity ratioand the phase difference.